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Silver soldering bronze

A bit of a disaster

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Martin Johnson 126/02/2019 08:31:19
128 forum posts
1 photos

I have just had to abandon four cylinder drain cocks, which were to be silver soldered assemblies. I had some "Unicast Holfos B" in stock for the bodies and had intended to silver solder in some brass connections. Suffice to say that after a couple of attempts the solder just does not want to wet the bronze. On the second attempt, I pickled the bronze bodies for a couple of days - after which they were BLACK. My chemistry is not great, but I wonder if Holos B contains lead?

Holfos B is an old specification (I can remember it but don't have any reliable information on the chemical analysis - but guessing the "....fos" is phosphor.

To avoid clogging this thread with the obvious suggestions, I have been model engineering for 50 years now, so have done a bit of silver soldering along the way.

So - are there certain grades of bronze that are best avoided for silver soldering? And if so, what others (apart from Holfos B) should be avoided, as quite a few contain lead? Or maybe this is just a one off disaster?

Hoping to be enlightened.


Michael Gilligan26/02/2019 08:38:36
14278 forum posts
628 photos

I think you are probably on the right track, Martin

Have a look at pp554-555 here: **LINK**'Holfos+B'&source=bl&ots=wOn34h5HSI&sig=ACfU3U130mPhMZ2h_xfRYnLnLjS6zQ2nqA&hl=en&sa=X&ved=2ahUKEwj51_eV_9jgAhVTtHEKHVrWDW8Q6AEwAHoECAQQAQ#v=onepage&q='Holfos%20B'&f=false


Edited By Michael Gilligan on 26/02/2019 08:39:49

not done it yet26/02/2019 08:42:51
3583 forum posts
15 photos

Can't help with your silver soldering. Only suggest using locktite (or equivalent) instead?

JasonB26/02/2019 08:45:41
16573 forum posts
1772 photos
1 articles

Michael's link shows no lead percentage,  SA660 and LG1 contain lead and they can be silver soldered so not sure if that is the reason.

Edited By JasonB on 26/02/2019 08:45:56

Edited By JasonB on 26/02/2019 10:41:41

CuP Alloys 126/02/2019 09:48:46
200 forum posts

Hi Martin.

Silver solder not wetting is always a flux problem. Silver solder not flowing can be due to heat pattern, joint gap, flux or contamination eg lead from a bronze.

Give us a call for more information. It's a quicker and easier way to communicate than the forum.

Looking forward to hearing from you.


Martin Johnson 126/02/2019 16:19:09
128 forum posts
1 photos

Michael G - Thanks for the link. It looks as though Holfos B is a tin bronze 88 Cu / 11.7Sn, with 0.7% P so it should be OK. I had looked on Google but you are clearly a better man than I, Gungadin.

But that leaves a mystery as to what had turned it black in my H2SO4 pickle. And as a consequence, the female threads which (should) hold the brass tails were not as clean as I would have liked, but I threw plenty of flux at it just in case.

Thanks Keith for that. I am trying to join quite small brass tails onto a relatively chunky bronze body - so that might be it. Anyway, I now need some more silver solder so will indeed give you a ring.

Thanks everybody,


fizzy26/02/2019 18:14:32
1626 forum posts
111 photos

You left it in the pickle too long...had it happen to me. No telling what went into making your bronze but it will and does turn black. Ive had brass do it as well. Get a bit of Tenicity 5 flux, mix it thick and it should work fine on clean metal. You need the extra temp flux due to the small piece of work getting hotter quicker. Good luck.

mark costello 126/02/2019 18:32:48
548 forum posts
12 photos

As I have followed  the forum for a while the problem of a flux not lasting long enough comes up regularly. Why even use the lower grade at all? Just go for the strongest at the get go.

Edited By mark costello 1 on 26/02/2019 18:33:13

Nigel Graham 216/03/2019 22:59:04
445 forum posts

Re original: "what other" bronzes to avoid!

Beware of Aluminium Bronze, widely used for marine equipment thanks to its very high corrosion resistance. Drills bind it for a pastime, but more pertinently here, it will not take conventional lead/tin and silver solders.

Polished, it has a rather paler than normal brass colour, which helps to differentiate it from other cuprous alloys.

How do I know? Guess! One of the hazards of the Pre-loved Metals bin...

CuP Alloys 121/03/2019 15:51:36
200 forum posts

Hi Martin.

At the risk of repeating myself, if the silver solder or soft solder doesn't "wet" you have a flux problem.

Use the right flux.

Aluminium alloys can be soft soldered.

Aluminium bronzes can be readily silver soldered.

Ring us for more information.


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